I need to fill native structs from managed classes. Following technique (1. copy managed data into managed byte array, 2. use memcopy to fill native struct) I found to be a common solution for this.
I assume the reason why following code doesn't work is that I use managed classes not managed structs. Having managed classes is a requirement in my project. Can I make this code work using managed classes, or must I switch to managed structs ?
this is the c# managed class:
[StructLayout(LayoutKind.Sequential)]
public class man_s
{
public man_s()
{
// (do something which i can't do in a struct!)
}
// should go into a one-byte native bool
[MarshalAs(UnmanagedType.I1)]
public bool flag1;
public Int32 a;
public Int32 b;
};
... the native cpp struct:
struct nat_s
{
public:
bool flag1;
__int32 a;
__int32 b;
};
... the code which should copy the managed data into the native struct:
// setup some managed data
man_s^ mng = man_s();
mng->flag1 = true;
mng->a = 10;
mng->b = 20;
nat_s nat;
int s = sizeof(nat);
// size check is ok!
System::Diagnostics::Debug::Assert(sizeof(nat) == System::Runtime::InteropServices::Marshal::SizeOf(mng));
// copy into managed byte array
array<byte>^ byteArray = gcnew array<byte>(s);
System::Runtime::InteropServices::Marshal::Copy(IntPtr((void*)(&mng)), byteArray, 0, s);
// this doesn't bring up the expected results
pin_ptr<byte> start = &byteArray[0];
memcpy(&nat, start, s);
// does not work either
System::Runtime::InteropServices::Marshal::Copy(byteArray, 0, IntPtr((void*)(&nat)), s);enter code here
Marshal.Copy is for copying data between managed arrays and unmanaged arrays. That's not what you have here: You have a managed object, and an unmnaged struct. For that, you want the PtrToStructure and StructureToPtr methods. Those methods target copying between a managed object and unmanaged memory of some sort.
// Despite the name, man_s is a managed class, not a managed struct.
// This means it gets the ^ (which you had correct),
// but it also means it gets gcnew (which you were missing).
man_s^ mng = gcnew man_s();
nat_s nat;
// You had this code is correct.
Debug::Assert(sizeof(nat) == Marshal::SizeOf(mng));
// StructureToPtr copies to unmanaged memory.
// An unmanaged array (i.e., allocated with `malloc` or `new byte[]`)
// would work, but a pointer to the unmanaged struct will also work just fine.
// The `false` means "Don't destroy the object that's already at the destination",
// which I believe does not apply here.
Marshal::StructureToPtr(mng, &nat, false);
// You can go the other way as well.
Marshal::PtrToStructure(&nat, mng);
// or
man_s = Marshal::PtrToStructure<man_s>(&nat);
Note: I'm not at a compiler right now. You might need to cast &nat to an IntPtr.
No. AFAIK you can't do that. You know nothing about the memory layout of a managed ref class. You may allocate an inner value type inside the class. You may pin this and copy this as a whole block.
Also I don't understand your code. You copy a managed memory with Marshal::Copy. And you do this in converting a managed class to a native pointer. Than you copy this into managed memory and copy this managed memory again to a native memory! Why at all? If you have a pin pointer that works as a native pointer.
Related
I have defined a struct MyStructure at IDL of unmanaged library SomeLibrary. I need to call managed code (C#) MyManagedLibPtr->RetrieveStuff from unmanaged library (C++) to fill and retrieve an array of these structs back for caller. The problem is that I haven't been able to figure out the signature of RetrieveStuff at managed side. I guess some custom marshalling is required? Here's what I have:
IDL of "SomeLibrary":
[
uuid(xxxxx-xxxx-xxxx-xxxx-xxxxx)
]
struct MyStructure
{
[helpstring("Some string values")] SAFEARRAY(BSTR) moValues;
[helpstring("Some other value")] BSTR moValue;
};
Unmanaged code (caller):
SAFEARRAY* saArray = NULL;
MyManagedLibPtr->RetrieveStuff(&saArray); // <--This is the key part
// The rest is just parsing the results.
// Using SafeArray -wrapper class that handles access/unaccess/etc..
SafeArray<SomeLibrary::MyStructure, VT_RECORD> oResults(saArray);
for (int i =0; i < oResults.GetSize(0); i++)
{
SomeLibrary::MyStructure oStruct = oResults[i];
// Etc......
}
At C# side, I've tried a few different solutions but none of them have been correct. This one would have been the sweetest, but obviously the mashalling automation wasn't sweet enough:
// Interface
[DispId(123)]
void RetrieveStuff(ref SomeLibrary.MyStructure[] roResultArray);
The error I get is some HRESULT -code. Haven't checked which one in particular, but obviously it's caused by incorrect signature or marshalling. Any help?
So, I found an answer that's ALMOST as requested. For whatever reason I couldn't get it working with ref parameter but I was able to do the marshalling with out. So this isn't 100% answer for the problem but damn close to it and will probably help others as well.
The correct interface declaration at managed side:
// Interface
[DispId(123)]
void RetrieveStuff([Out, MarshalAs(UnmanagedType.SafeArray, SafeArraySubType = VarEnum.VT_RECORD)] out SomeLibrary.MyStructure[] roResultArray);
Consider the following stucture in c++, I need to wrap this using CLI to make it usable for C#
typedef struct A
{
int b[5];
A* next;
};
How can I do so? Please help.
non-op edit:
What I've tried is the following:
public ref struct ANET
{
array<int>^ b;
ANET^ next;
ANET()
{ b = gcnew array<int>(5) }
};
now let's say we have an pointer of object of A is a, pointer of object ofANET is aNET
nSize = sizeof(a->b)/sizeof(a->b[0]);
Marshal::Copy( IntPtr( ( void * ) a->b ), aNET->b, 0, nSize);
so far I am able get the values of a->b in aNET->b but I am stuck with marshaling the pointer of a->next to aNET->next.
Thanks
That's a linked list. So one option is to write C++/CLI code to adapt the native linked list to a .net LinkedList<T>. Going down this route will mean that you need to make copies of the list whenever it is modified in the native code and needs to be returned to the managed code. And vice versa.
If making copies is not an option then you should wrap the native linked list in a managed class. So you don't try to convert the struct definition at all. Instead you write a C++/CLI managed class that encapsulates the functionality that you need to expose. I can't say much more than that because you haven't given any information on the operations that you need to perform on this list. Now have you explained what the data flow is.
I have been looking all over google to find some answers to my questions but do not quite understand what I have found. I have some objects which are created and stored in C# List after using System.IO to read some text files. After that, I want to send references (using const pointers) to each of these objects to the internal classes in C++ dll so that it can use them for computation of some algorithms.
Here are some simple example (not actual code) of what I am doing:
The C# class:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public class SimpleClass
{
[MarshalAs(UnmanagedType.LPStr)]
public string Name;
public float Length;
}
with corresponding C struct:
struct SimpleClass
{
const char* Name;
float Length;
};
stored in
List<SimpleClass> ItemList;
after parsing some text files.
Then calling the following dll function:
C#:
[DllImport("SimpleDLL")]
public static extern void AddSimpleReference(SimpleClass inSimple);
C:
void AddSimpleReference(const SimpleClass* inSimple)
{
g_Vector.push_back(inSimple); // g_Vector is a std::vector<const SimpleClass* > type
}
What I have tried is:
for(int i=0; i<ItemList.Count;++i)
{
SimpleClass theSimpleItem = ItemList[i];
AddSimpleReference(theSimpleItem);
}
Initially, I thought it would be easy to get a actual reference/address just by using the assignment operator since classes in C# are passed-by-reference but it turns out that the C++ class is always pushing the same address value (the address value of the temp reference) into the container instead of the actual address. How do I get the actual object addresses and send it to C++ DLL so that it can have read-only access to the C# objects?
UPDATE: Sorry to those who posted answers with unsafe codes. I forgot to mention that the C# code is actually used as a script in Unity game engine which does not allow the use of unsafe codes.
First you need to change your interop signature to take a pointer (and thus making it unsafe).
[DllImport("SimpleDLL")]
public unsafe static extern void AddSimpleReference(SimpleClass* inSimple);
Then, because the GC is free to move objects around in memory as it pleases, you will need to pin the object in memory for the entire time you will need its address on the unmanaged side. For that you need the fixed statement:
SimpleClass theSimpleItem = ItemList[i];
unsafe
{
fixed(SimpleClass* ptr = &theSimpleItem)
{
AddSimpleReference(ptr);
}
}
This would work if AddSimpleReference used the pointer and then discarded it. But you're storing the pointer in a std::vector for later. That won't work, because the pointer will probably become invalid due to the GC moving the original item somewhere else once execution leaves the fixed block.
To solve this, you need to pin the items until you are done with them. To do this you may need to resort to the GCHandle type.
// Change the interop signature, use IntPtr instead (no "unsafe" modifier)
[DllImport("SimpleDLL")]
public static extern void AddSimpleReference(IntPtr inSimple);
// ----
for(int i=0; i<ItemList.Count;++i)
{
SimpleClass theSimpleItem = ItemList[i];
// take a pinned handle before passing the item down.
GCHandle handle = GCHandle.Alloc(theSimpleItem, GCHandleType.Pinned);
AddSimpleReference(GCHandle.ToIntPtr(handle));
// probably a good idea save this handle somewhere for later release
}
// ----
// when you're done, don't forget to ensure the handle is freed
// probably in a Dispose method, or a finally block somewhere appropriate
GCHandle.Free(handle);
When doing something like this, keep in mind that pinning objects in memory for a long time is a bad idea, because it prevents the garbage collector from doing its job efficiently.
Even though I think this is not a good idea, have a look at unsafe code and memory pinning. Here is a good start on MSDN.
fixed and unsafe keywords are likely what you should be looking for.
You cannot. The C# GC will move objects for fun. Your addresses will go out of scope.
My gut reaction is no, because managed and unmanaged memory are distinct, but I'm not sure if the .NET Framework is doing something with Marshaling behind the scenes.
What I believe happens is:
When getting a struct from my unmanaged DLL, it is the same as making that call gets an IntPtr and then uses it and the Marshal class to copy the struct into managed memory (and changes made to the struct in managed memory do not bubble up).
I can't seem to find this documented anywhere on MSDN. Any links would be appreciated.
Here is what my code looks like:
[DllImport("mydll.dll", BestFitMapping=false, CharSet=CharSet.Ansi)]
private static extern int GetStruct(ref MyStruct s);
[StructLayout(LayoutKind.Sequential, Pack=0)]
struct MyStruct
{
public int Field1;
public IntPtr Field2;
}
public void DoSomething()
{
MyStruct s = new MyStruct();
GetStruct(ref s);
s.Field1 = 100; //does unmanaged memory now have 100 in Field1 as well?
s.Field2 = IntPtr.Zero; //does unmanaged memory now have a NULL pointer in field Field2 as well?
}
No, the P/Invoke marshaller copied the unmanaged structure member values into the managed version of the structure. In general, the managed version of a structure is not in any way compatible with the unmanaged version of it. The memory layout is not discoverable, something the CLR uses to reorder fields to make the structure smaller. Marshaling is essential, you have to create a copy.
Modifying the structure is not possible with the given function signature since you let fill in the memory that's passed to it. The function itself already copies the structure. You can however party on the Field2 value since it is a raw pointer. If that points to a structure then marshal it yourself with Marshal.PtrToStructure(). Modify the managed copy of it and copy it back to unmanaged memory with Marshal.StructureToPtr(). Or access it directly with Marshal.ReadXxx() and WriteXxx().
CSharp Language Specification.doc pg 26
Struct constructors are invoked with the new operator, but that does not imply that memory is being allocated. Instead of dynamically allocating an object and returning a reference to it, a struct constructor simply returns the struct value itself (typically in a temporary location on the stack), and this value is then copied as necessary.
Since, there is nothing special about a 'struct' backing store, so one would not expect there to be annonymous marshalling operations going on behind the member assignments.
I'm calling functions from C++ that returns a pointer to an array of struct and I'm having problems since I'm new to this operation/implementation.
My C++ codes:
// My C++ Structs
typedef struct _MainData {
double dCount;
DataS1 *DS1;
int iCount1;
DataS2 *DS2;
int iCount2;
}MainData;
typedef struct _DataS1 {
unsigned int uiCount1;
unsigned int uiCount2;
int iCount;
void *pA;
void *pB;
} DataS1;
typedef struct _DataS2 {
unsigned int uiCount1;
unsigned int uiCount2;
unsigned int uiCount3;
unsigned int uiCount4;
double dCount;
int iCount1;
char strLbl[64];
} DataS2;
// My C++ Function
MainData* GetData(const int ID)
{
MainData* mData;
int iLength = Get_Count();
mData = new MainData[iLength];
for(int x = 0;x < VarCounter; x++)
{
// Codes here assign data to mData[x]
}
return mData;
}
Question:
How can I call the C++ function GetData to C#?
My current codes in C# are:
[DllImport(".\\sdata.dll")]
[return: MarshalAs(UnmanagedType.LPArray)]
private static unsafe extern MainData[] GetData(int ID);
// The struct MainData in my C# side is already "Marshalled"...
//My function call is here:
MainData[] SmpMapData = GetData(ID);
When I compiled it, there's an exception:
"Cannot marshal 'return value': Invalid managed/unmanaged type combination."
Sorry for the poor coding... Please help...
First, you need to remember that MarshalAs (explicit or implicit) for the return value essentially means "copy native structures content into managed structures".
Second, since the CLR marshaler only copies the data, if you do not free the memory you're allocated in the C++ function, you've got a memory leak to manage.
Third, this error is mainly due to the fact that the CLR marshaler has no way of knowing the length of the array returned by the native code, since you're basically returning a memory pointer and no length.
If you want to keep these memory structures as-is, I strongly suggest you to look into C++/CLI. You'll be able to wrap those complex types into mixed native/managed classes that will avoid you to copy the data around. This will help you keep the data marshaling to the bare minimum between native and managed code.
If you still want to use C# and no C++/CLI, you'll have to write a somehow smarter piece of code to unmarshal the data returned by the native code into managed data. You can look into Custom Marshaling for that.
I don't see how the .NET runtime could possibly know how many MainData are allocated in GetData(...).
Refactor your C++ code to consume an array to populate or return single MainDatas.